State-of-the-art in passive seismic array methodology, And useful application for mineral exploration. Nick Smith PassiveX Pty. Ltd.

Transcription

1 State-of-the-art in passive seismic array methodology, And useful application for mineral exploration Nick Smith PassiveX Pty. Ltd.

2 l Microtremor wavefield l Why use arrays? l Array techniques l State of the art methodology l

3 Microtremor Central Australia North Chile Microtremor Microseism

4 Surface Waves Offset Time Body waves Decay = 1/r2 Surface waves Decay = 1/r

5 Surface Waves Bensen et.al. (2007)

6 Surface Waves Sensitivity Love Waves Rayleigh Waves Depth 10 Hz 5 Hz 1 Hz

7 Surface Waves Surface wave phase velocity sensitive to seismic velocity structure (Vp, Vs and Density) l Most sensitive to Vs l Vs insensitive to fluids (e.g. groundwater)

8 Why Use Arrays? l Fundamental information on cover! Cover +/- bedrock velocity structure Stratigraphic variability

9 Array concepts: l Seismic interferometry Retrieve coherent wavefield propagating between two sensors Greens function l Array beamforming Delay and sum, power distribution composition, directionality and velocity

10 Array SPAC l Circular array, 4+ sensors l Ave. coherency for sensor pairs l Independent of wavefield directionality l Surface wave dispersion l Shear-wave velocity vs depth.

11 Array SPAC l Cross correlation / coherency l Isotropic wavefield OR azimuthal average Coherency Coherency Azim. Ave. coherency Frequency

12 Array SPAC l Sensor separation wavelength sensitivity r1 r2 r3 r4

13 Array FK l Complex array design, 10+ sensors l Array beamforming l characterisation l Velocity vs depth

14 Array FK l Array design and array response Kennett, et.al. (2015)

15 Array FK l Array response steering by coherent signal Basic concept: delay and sum of signals In practise: calculate power for different combinations of back azimuth and slowness

16 Array FK Kennett, et.al. (2015) PSAR SQspa

17 Array FK Gal, et.al. (2014)

18 Array State of the art methodology l Three component constraints Sensitivity Love Waves Rayleigh Waves Depth

19 Array State of the art methodology l Inversion techniques e.g. Neighbourhood Algorithm e.g. Markov Chain Monte Carlo NA Velocity versus Depth

20 Array State of the art methodology l Markov Chain Monte Carlo Bayesian inference Correct propagation of uncertainty into modelling result Does not require explicit regularisation or parameterisation Trans-dimensional

21 Array State of the art methodology l Markov Chain Monte Carlo Bodin, et.al. (2012)

22 Turning noise into signal is really interesting BUT.. Why is this important for mineral exploration?

23 1. Shear wave velocity vs depth profiles for greenfields passive seismic work 2. Assessment of the changeability of 1) throughout the project area 3. Assessment of complex geology 4. Assessment of complex wave propagation phenomena for best modelling results

24 1) and 2): Average Vs variation - H/V resonance frequency depth conversion - Geological structure

25 3) Complex geology - Multiple large impedance contrasts - Velocity inversion Azi. Ave. Coh SPAC SPAC + H/V Stratigraphy Ellipticity Depth (m) Frequency (Hz) Vs (m/s)

26 4) Complex wavefield - Higher modes - Sites with large Vs contrasts, or velocity reversals - Source characteristics n = 1 n = 2 n = 4 n i V < ni+1 V < ni+2 V...

27 Higher modes SPAC using model based effective mode and amplitude response for assumed harmonic vertical point sources Ikeda, et.al. (2012)

28 Summary Passive seismic arrays are not just interesting... They are also useful for: l Characterising the wavefield l Characterising the geology l Validating assumptions And are critical for l Accurate and adaptive large scale passive seismic surveying